ISO: PDP-11/40 LTC and Stack Limit options

Noel Chiappa jnc at
Tue Nov 22 11:27:25 CST 2016

    > From: Josh Dersch derschjo at 

    > see if the same is true for other bus grants -- I can run the system
    > with no grant continuity card at all in slot 9 and everything works.

Well, the BG4-BG7 grants definitely _are_ run through the SPC slot 9 (see
below) - at least, on a stock system. It's _possible_ that the software
you're loading doesn't use interrupts. (I have this vague memory that, unlike
the -11/34, the /40 doesn't complain if there's a non-continuous grant line.)
Or perhaps someone wired them across on that slot, to avoid knuckle-mashing
trying to put a G727 down there.

Anyway, the wire list in the drawings show all four lines (although they are
listed in two places, under "BGx" and "BUS BGx"). E.g. BG4 is shown on pg. 79
as going from D07E2 (Source - K4-6, pg. 63, top right) to D09S2 (which is the
correct BG4 'in' pin for SPC), and as BUS BG4 on pg. 84 as going from D09T2
(SPC BG4 'out' pin) to B09E2 (correct BG4 UNIBUS 'out' pin).

    > I now have the system booting XXDP


    > I did find out why there was that wire missing on the backplane; the
    > KW11-L requires a wire (carrying one of the bus grant signals) be
    > removed from slot 3.

Right, BG6 is wired through that KW11-L slot because the clock needs
interrupts - the wire list shows that on pg. 79, where the BG6 entry is longer
than the other BGn entries, because of that. If I'm reading the notations
correctly, it shows the jumper installed by default - I guess it was removed
by hand on systems sold with a KW11-L?

There must also be some way to indicate that the jumper should be wired on
top at both ends (so the F03V2 to D09M2 wire wouldn't have to be removed to
pull the F03R2 to F03V2 jumper) - although maybe they just did _all_
multi-pin runs as alternating low on both ends, high on both ends, repeat to
make removal/replacement easier.

Speaking of notation, dunno if you knew this (I didn't), but the wire list
for the 11/40 includes etch also; you can tell etch entries from an 'H' in
the "Q" column and 'P' in the "Remark" column. Don't confuse them with the
'H' in the "A/P" column, which also also has some 'L' entries; not sure what
that is about, unless it tells whether the signal is asserted high or low.


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